Plastic fuel tanks are commonly used in automobiles because of their strength, formability, weight and cost. One requirement of all automotive fuel tanks is the ability to withstand exposure to the high temperatures that may be encountered under a vehicle. Current plastic automotive fuel tanks are made from high density polyethylene (HDPE). HDPE has a glass transition temperature of LESS THAN 300.degree. F. This is normally sufficient for most automotive applications. However, some applications require that the fuel tank withstand exposure to higher temperatures.
When the fuel tank is placed near an exhaust manifold or catalytic converter, it must generally be shielded from higher temperatures. Current methods of shielding plastic fuel tanks include attaching fasteners to sockets formed in the tank as illustrating in U.S. Pat. No. 5,067,575. Mechanical fasteners are currently the preferred choice for attaching heat shields to plastic tanks because HDPE is not retentive to adhesives.
It is also know to manufacture blow-molded fuel tanks with a fabric outer layer for additional strength as taught in U.S. Pat. Nos. 5,020,687 and 5,194,212. These patents teach placing a mat of nylon or glass fibers within the interior of a mold. A parison is extruded adjacent the mat and inflated. The pliable parison penetrates the strands of the mat and secures the mat to the exterior of the tank. Plastic or liquid resin impregnates the mat to fasten it to the tank. The mat may provide some degree of temperature insulation.
Insert molded fiber reinforcement generally does not provide sufficient heat shielding for high temperature locations. Fuel tanks as exemplified by U.S. Pat. No. 5,020,687 generally require the parison to completely permeate the mat. The mat is usually made from compressed non-woven fibers. Lose or stray fibers may be easily shredded or torn from the mat. Impregnating the mat with the parison encases the fibers in plastic and prevents stray fibers from being torn. Impregnating the mat also significantly reduces its insulating properties. Air trapped within the mat provides it with insulating properties. The glass or nylon fibers are generally thermally conductive. When the mat is completed impregnated with plastic, it looses most of its insulating properties. Mats attached in this fashion retain little if any of their insulating properties.
Fuel tanks as exemplified in U.S. Pat. No. 5,194,212 similarly displace all of the air from the mat. High temperature resistant resin is used to give the fuel tank greater strength under exposure to high temperatures. Although this design can withstand the higher temperatures and remain structurally sound, it does not provide the insulating properties of the present invention. The resin impregnated mat become a thermal conductor. Heat is transferred through the mat and to the inner plastic wall and fuel within the tank.